I'm a sucker for good line-art and Brad "BKM" McDevitt adorns this adventure with some exquisite point-of-view pictures (think D&D Tomb of Horrors player pull-out).
I'd say Dream of Japan comes pretty close to being a perfect CoC mystery. My only issue is that, like with nearly all CoC adventures, it needs a fairly resourceful Keeper to usher the investigators along the right path. However, in this adventure there is a fail-safe - the investigators have been manipulated, possibly since birth (!) by unseen forces, so the Keeper now has a licence for contrivance. ;)
This adventure is a perfect opportunity to plunge the players into a superstitions world, that's just alien enough (the Orient) to make the investigators paranoid about every lucky penny they find. This adventure looks like it has the makings to be a classic - and perhaps even a whole campaign. It has some really nice twists, great art (the maps are good too).
Designed for Chaosium/BRP Call of Cthulhu (5) but could be easily adapted or sourced for other games set in the '20's.